A variable displacement reciprocating compressor used in an automotive air conditioning system, for example, has a housing, and inside the housing, a discharge chamber, an intake chamber, a crank chamber and cylinder bores are defined. On a drive shaft extending inside the crank chamber, a swash plate is mounted to be variable in inclination, and a conversion mechanism including the swash plate converts rotation of the drive shaft into reciprocating motion of pistons fitted within the respective cylinder bores. By the reciprocating motion, each piston performs a discharge process of drawing an operating fluid from the intake chamber into its own cylinder bore, compressing the drawn-in operating fluid and discharging the compressed operating fluid to the discharge chamber.
The stroke of the piston, therefore, the displacement, or amount of the operating fluid discharged per reciprocation is varied by varying pressure in the crank chamber (control pressure). In order to control the displacement, a displacement control valve is arranged in a gas supply passage connecting the discharge chamber and the crank chamber, and a constriction is provided in a gas release passage connecting the crank chamber and the intake chamber.
The displacement control valve is controlled by a control device. For example, a control device disclosed in Japanese Patent Application KOKAI Publication 2001-132650 (Patent Document 1) performs feedback control of displacement in a manner causing a pressure difference (differential pressure) between pressure in the discharge chamber (discharge pressure) and pressure in the intake chamber (intake pressure) to approach a target. Specifically, the control device of Patent Document 1 controls the pressure difference by varying current supplied to the displacement control valve, as a result of which, the displacement varies. For example, the operation of the control device decreasing the differential pressure to maintain it at a specified value results in an increase in displacement.
Japanese Patent Application KOKAI Publication Hei 9-268973 (Patent Document 2) and Japanese Patent Application KOKAI Publication Hei 11-107929 (Patent Document 3) each discloses a displacement control valve with a pressure sensing member incorporated therein, where the displacement is feedback-controlled on the basis of the intake pressure sensed by the pressure sensing member. The pressure sensing member consists of, for example a bellows, which expands to increase the flow passage area in the gas supply passage to decrease the displacement, when the intake pressure decreases.
However, for example when the refrigerant quantity is insufficient in the automotive air conditioning system, the displacement control on the variable displacement compressor by the control device disclosed in Patent Document 1 results in a smaller differential pressure between the discharge chamber and the intake chamber, compared with when the refrigerant quantity is adequate.
In such case, in order to cause the differential pressure to approach the target, the control device of Patent Document 1 operates to increase the displacement. Thus, when the refrigerant quantity is insufficient, operating the variable displacement compressor by feedback-controlling the differential pressure results in accelerated increase of the displacement, since the differential pressure does not reach the target. Eventually, the compressor continues operating with the maximum displacement, which may lead to breakage of the compressor.
Even in such case, if feedback control of displacement is performed by sensing the intake pressure by the pressure sensing member, as in Patent documents 2 and 3, the risk of the compressor breakage reduces. This is because when the insufficient refrigerant quantity causes decrease of the intake pressure, the displacement is decreased in order to maintain the intake pressure at a specified value, where the displacement is decreased finally to its minimum. In this sense, the feedback control of displacement by sensing the intake pressure includes a fail safe function.
The feedback control described in Patent Documents 2 and 3, however, requires that the displacement control valve include a sensing member for sensing the intake pressure. Specifically, the sensing member includes a bellows, a diaphragm or the like defining a closed space variable in volume and being vacuum or at atmospheric pressure. The use of such sensing member leads to complex structure of the displacement control valve.
Further, when heat load on the refrigeration cycle of the air conditioning system is great and revolving speed of the compressor is low, control of the intake pressure using the displacement control valve including a pressure sensing member may not produce a sufficient decrease of the displacement. In such case, actual intake pressure may exceed the range of control, resulting in complete loss of control over displacement. The loss of control over displacement requires stop of the compressor, which damages the conditioned state of air in the vehicle interior.
For example, driving the variable displacement compressor is a great load for the engine of the vehicle. Thus, there is taken a measure such that while the vehicle is accelerating or moving up a slope, for example, the displacement is temporarily decreased to decrease the compressor driving load. This is to apply as much engine power as possible to the vehicle's running, while maintaining a certain level of air conditioning performance. In such case, however, a great heat load leads to loss of control over intake pressure, which requires stop of the compressor, thus greatly damaging the conditioned state of air in the vehicle interior.
Further, in order to protect the compressor and the air conditioning system, there is taken a measure such that a discharge pressure sensor detecting the pressure of a refrigerant is provided in the high-pressure side of the refrigeration cycle to perform control causing decrease of the displacement when the pressure detected by the discharge pressure sensor exceeds a set threshold. However, the discharge pressure exceeding the threshold may entail an intake pressure exceeding the controllable upper limit. In such case, the compressor needs to be stopped, which greatly damages the conditioned state of air in the vehicle interior.
These problems are connected with the fact that the maximum intake pressure controllable by the displacement control valve including the bellows is low. Specifically, FIG. 2 of Patent Document 3 shows a relationship between pressure in the intake chamber and current supplied to the solenoid, where the refrigerant is R134a. The controllable upper limit of intake pressure is between 0.3 and 0.4 MPa. In order to make the displacement control practicable even with a great heat load, it is necessary to raise this controllable upper limit, thereby widening the range of control of intake pressure to a great extent.
The range of control of intake pressure can be widened by increasing the electromagnetic force generated by the solenoid. The significant widening of the range of control however entails an increase in size of the solenoid, which is undesirable in view of design rationality.
Another conceivable means of widening the range of control is forming the bellows with a reduced size so that the bellows has a pressure-sensing surface sensing the intake pressure, reduced in area (effective area). The reduction in size of the bellows is however restricted by the necessity to provide a coil spring and a stopper restricting the expansion and contraction of the bellows, inside the bellows vacuum or at atmospheric pressure.
Further, when a diaphragm is used to sense the intake pressure in place of the bellows, reduction in area of a pressure-sensing surface of the diaphragm entails reduction in displacement of the diaphragm, thus reduction in stroke of the valve, in order to ensure sufficient life of the diaphragm. The reduction in size of the diaphragm is therefore restricted by this requirement.
Further, in the air conditioning system using carbon dioxide as a refrigerant, the refrigerant pressure becomes very high, which makes it difficult to feedback-control the displacement by sensing the intake pressure by means of a pressure-sensing member such as a bellows.
Further, it is demanded that in the vehicle's idling, the load on the variable displacement compressor be regulated to allow stable control of the engine revolving speed. So far, such regulation has not been achieved by the displacement control method controlling the intake pressure.